Depending upon the particular process (which may include a diffusion, annealing or deposition step), wafers are treated at temperatures of from about 250.degree. C. to about 1300.degree. C. under very closely controlled conditions in rather sophisticated furnaces. U.S. Pat. No. 4,348,580 discloses a typical horizontal furnace, and U.S. Pat. No. 4,547,404 discloses a typical vertical furnace. Such furnaces have resistance heating elements disposed about the outside of the processing chamber for heating the wafers and gases in the processing chamber to a processing temperature. As these patents indicate, wafers are placed in the so-called "flat temperature zone" of the processing chamber where the temperature can be controlled by +0.5.degree. C. of the temperature set point according to U.S. Pat. No. 4,348,580 or the temperature differential in the zone can be held to less than 2.degree. C. according to U.S. Pat. No. 4,547,404.
Although the temperature across the horizontal length of the flat zone may be controlled within the specified limits in commercial furnaces, the top to bottom temperature along the vertical height of the flat zone is not monitored. Rather, gas flow is assumed to maintain a reasonably constant (but undefined) process temperature in the annular space between the wafers and the facing wall of the processing chamber. The fact is, however, that the annular temperature around the wafers may be substantially different and this difference may vary significantly depending upon the process temperature. Notwithstanding the flow of gas through the processing chamber and the turbulence induced by the internal configuration of the processing chamber in the gas, thermal convection currents in the narrow annular space between the wafers and the wall of the processing chamber have an unexpectedly significant effect. The temperature differential is particularly severe in low temperature processes in 250.degree. C. to 450.degree. C. range where radiation is not as significant a factor as it is above about 450.degree. C.
The top to bottom temperature differential in the flat zone inherent in the prior art practice was dramatically shown in tests summarized by FIG. 1. The tests were conducted in a horizontal diffusion furnace comprising a conventional quartz processing tube having an internal diameter of 225 mm and length of about 1.2 meters. A boat loaded with 50 6" wafers was placed in the flat temperature zone of the tube which is the central zone between the load end zone and the source end zone. The tube was maintained at about atmospheric pressure with about 10 standard liters per minute nitrogen continuously flowing through it. The tube temperature was controlled by a helically formed electrical resistance element positioned coaxial with and encircling the tube with the load, central and source temperature zones being connected in series with each zone controlled in response to the temperature monitored under the portion of the boat in or nearest such zone. The nominal temperature in the atmosphere of the central zone was measured by a thermocouple located near the boat's midpoint ("T bottom"). The temperature at the top of the central zone ("T top") was measured by a second thermocouple located above the wafers vertically above the thermocouple under the boat. The temperature differential in the central zone was calculated as the difference between the temperatures sensed by these two thermocouples.
FIG. 1 indicates the steady state temperature differential (T top-T bottom) in the central temperature zone as a function of the process temperature (T bottom). Curve 10 shows that there is a steady state top to bottom temperature differential inherent in the prior art. As curve 10 shows, the differential in the above described furnace was about 5.degree. C. at a process temperature of 250.degree. C. and decreased with increasing temperature. The temperature differential did not come within 1.degree. C. of the process temperature until the process temperature was about 1000.degree. C.